Thyratron control system



July 26, 1960 Filed July 1, 1955 2 Sheets-Sheet 1 THE L THYRATRON DOMEDRIVE -2 CONTROL DRIVE APPARAJUS E plRCUiT APPARATUS O r 27 2a IO CTTHEODOLITE l5 DRIVE u CONTROL s 400 CYCLE SUPPLY 's|e.(d-c) FIG. 3

+ SlG.(d-c) CRITICAL VOLTAGE Ed-c O L I e firing point at null WOT bmsINVENTOR firing point out of null eo-c WALTER R. PECK BY 6/50,, fliwvw/(wi,

ATTORNEYS W. R. PECK THYRATRON CONTROL SYSTEM July 26, 1960 2Sheets-Sheet 2 Filed July 1, 1955 F2516 wmw JON; ZOO ZONE/m CC. @QIRHEIIIRM IIIII INVENTOR WALTER R. PECK BY 6;, flo w 015 ATTORNEYS nitedStates Patent THYRATRON CONTROL SYSTEM Walter R. Peck, Asheville, N;C.,assignor to Amcel Propulsion, Inc.

Filed July 1, 1955, SenNo. 519,390

12 Claims; (Cl."318-293) only when its control grid voltage reaches acritical level,

and of continuing conduction untilits plate voltage decreases tosubstantially zero. As a result of this characteristic, the thyratronissubject to rather precise control, and the portionof a cycle of platevoltage within which the tube conducts can bexvaried from cycle tocycle.

One of the problems oftenmetin design of thyratron control circuits isthat thelsignalvoltage which is to be used to controlfiringofthethyratron often is of a different frequency or phase from'thefrequency and phase of the plate voltage. For instance, when synchrosystems or servo-mechanisms, are used to control the firing ofthyratrons, thesignal voltage of the synchro which is applied to thethyratron circuit is ordinarily of a different frequency than -thesupply mains voltage used as a source of plate voltage for thethyratron.In such a situation, if the control or signal voltage were applieddirectly to the thyratron grid, the conduction time would Vary fromcycle to cycle out of the control of the operator or of thecontrollingdevice. Since'this is obviously disadvantageous, several solutions tothe problem of making the signal voltage and the thyratron plate voltageof the same or of controllable phase relation, have been proposed. Oneofthese solutions is'to rectify the signal voltage, and then convert theresulting D.-C. voltage into an A.-C. voltage by use of a socalledchopper, the latter being controlled by the same source of voltage whichsupplies the thyratron plate. This solution, and the others of whichapplicant is aware, require rather extensive 'and'expensive additions tothe thyratron control circuits; The present invention is designed toavoid following the solutions previously proposed, and to "decrease thecost and complexity of the thyratron control circuit.

The method of the present invention includes the steps of obtaining anAz-C. signal voltage, comparing this A.-C. signal voltage with an A.-C.reference voltage having the same frequency as'the signal voltage-and ofconstant phase toiobtain a comparison voltage, rectifying the comparisonvoltage to obtain a DC. signal voltage, and combining with :the -D.-C.signal voltage and A.-C. bias voltage of constant frequency andcontrollable phase relation with the A.-C. plate voltage of thethyratron, and-also. combining withthe A.'-C. bias and D.-C. signalvoltages a 11-0 bias voltage to set the D.- C. bias of the thyratrongrid ataan'appropriate level below the critical voltage. Thus, the onlyA.-C. volt- 2,946,942 Patented July 26, 1960 age which is supplied tothe grid of the thyratron is one which has a constant, or controlledphase relation with the plate voltage, yet it is not necessary to gothrough the complex procedure of, for instance, rectifying the signalvoltage and then converting it back into an alternating current voltageof-constant phase relation to the plate voltage.

The present invention is particularly useful in conjunction with anapparatus disclosed in a co-pending application filed by Edward L.Morgan, entitled Protective-Housing, and'assigned to the same assigneeas the present invention. The Morgan application has been given SerialNumber 521,730, and was filed July 13, 1955, now Patent No. 2,846,962.The Morgan application discloses a protective housing or'do'me for aphototheodolite. Reference is made to the Morgan application for a fullunderstanding of the features of the protective housing, and suffice itto state herein that the Morgan apparatus includes a motor drive systemwhich is designed to drive a pinion in mesh'with a gear carried by thehousing. The housing is caused thereby to rotate with the motor, and,through use of a circuit to be described specifically in thisapplication, the motor for the housing is caused to track with the motorwhich drives the theodolite.

The method and'apparatus of the present invention will now be morefully'desc'ribed in conjunction with a specific embodiment'of theinvention shown in the accompanying drawings.

In the drawings:

Fig. l is a schematic diagram, partly in block form, showing therelative connections between the theodolite drive apparatus,- thethyratro'n control circuit, and the dome drive apparatus;

Fig. 2 is a schematic diagram of the elements of the thyratron controlcircuit-of Fig. 1; and

Fig. 3 is a graphic representation of the control of one of thethyratrons of Fig. 2, showing the control grid voltage incomparison'with the platevoltage of the thyratron.

In Fig. 1, a'drive motor 1 rotates a theodolite drive apparatus 2 tocause movement of a photo-theodolite (not shown). Reference is made toan article titled New'Tracker Pinpoints Missiles, Aviation'Week,September l3, l954,'for a disclosure of the essential characteristi'csof the photo-the'odo'lite.

Drive motor 1 is controlled by a theodolite drive control circuit 3which need not be specifically described in this application. Sufiice itto state here'th'at the drive control circuit 3 causes movement of theshaft of motor 1 to turn the theodolite drive apparatus and therebyrotate the theodolite. Since the viewing means and camera of thetheodoliteproiect through slots in the protective housing of the Morganapplication, it is desired to rotate the housing in synchronism withrotation of the theodolite itself. A circuit to accomplish this effectwill now be described.

The shaft of motor '1 drives the rotor of a control transmitter typesynchro unit 5 identified with the letters CX in Fig. l. The rotor coilof the synchro is supplied with 400 cycle voltagefrom a supplyidentified at 6 in Fig. 1. As a result of this supply and the drivingconnection, the three stator coils 7, 8 and Q, of the synchro developvoltages thereacross whose relative magnitudes and polarities uniquelydefine the angular position of the shaft of the synchro with respect tothe stator thereof. Stator coils 7, 8 and 9 are connected tocorresponding stator coils 10, 1 1 and 12, of a control transformer typesynchro 15,identified as CT in Fig. l.

The apparatus which it is desired to control in synchronism with thetheodolite drive apparatus 2 is identified in block format20 in Fig. 1and labelled Dome This apparatus is driven by a motion of rotation ofthe motor 21 and of the extent of movement thereof is had by use of athyratron control circuit identified at 25 and shown in block form inFig. 1. Motor 21 drives the control transformer synchro 15, thus causinga voltage to be developed across rotor coil 26 of the synchro which isproportional to the sine of the diiference between the angle defined bythe voltages supplied to its stator leads, and the angular position ofthe shaft of the synchro with respect to its stator. This voltage issupplied to the thyratron control circuit across leads marked 27 and 28,the voltage thereacross being labelled E in Fig. 1.

As will be explained hereinafter, as part of the procedure ofcontrolling the thyratron control circuit 25 of Fig. 1, the signalvoltage E is compared with a reference voltage Eqef, having the samefrequency as the signal voltage and of constant phase. This voltage isderived from the 400 cycle supply 6 of Fig. 1 and is connected to thethyratron control circuit by leads 29 and 30.

With the apparatus of Fig. 1, as long as the thyratron control circuitsupplies an output current for the motor 21 which varies in value and inphase with the extent and direction of movement of motor 1, motor 2'1will be driven in synchronism with motor 1.

Referring now to Fig. 2, the thyratron control circuit 25, morespecifically, includes a pair of thyratrons 35 and 36. The circuit tosupply an appropriate control voltage for the grids 37 and 38,respectively, of the thyratrons, will now be described. The A.-C. signalvoltage of 400 cycles, E is supplied across leads 27 and 28, which areconnected across a potentiometer 40 havconducted to the thyratroncontrol circuit by a pair of leads 29 and 30. Lead 29 is connectedthrough a-pair of voltage-dropping resistances and 51, to grid 52 of apentode vacuum tube 53. Lead 30 is connected through a similar pair ofvoltage-dropping resistances 54 and 55 to the control grid 56 of anotherpentode 57. A potentiometer 58 is connected between the junction pointsof the pairs of voltage-dropping resistances, and the slider 59 of thepotentiometer is connected to ground. The cathodes 60 and 61 of thepentodes are connected through the conventional R-C bias circuit 62 toground. The screen and suppressor connections of the pentodes need notbe more fully described, since they are of conventional type. As aresult of these connections, a voltage of one phase varying inproportion with the A.-C. reference voltage is developed between plateand cathode of pentode 53, and a voltage of the opposite phase, likewisevarying with the reference voltage, is developed between plate andcathode of pentode 57.

In order to permit comparison of the output voltages derived fromtriodes 44 and 45, and pentodes 53 and 57, a pair of transformers 65 and66, having center tapped primary windings 67 and 68, respectively, isprovided. Plate 69 of triode 44 is connected to one lead 70 of primarly67, while plate 71 of pentode 53 is connected to the opposite lead 72 ofthe primary. Also, plate 75 of pentode 57, and plate 76 of triode 45,are connected to opposite leads, 77 and 78,, respectively, of primary68.

The center taps of the two primaries are connected together by lead 80and are connected to a suitable source of plate voltage source 80 whichsource is, as usual, connected between plate and ground of each of thefour vacuum tubes.

Each of transformers 65 and 66 has a secondary, 81 and 81',respectively, so that an A.-C. voltage proportional to the differencebetween the A.-C. signal voltage E and the A.-C. reference voltage E ofone phase is developed across secondary 81, while an A.-C. voltageproportional to the diiference between ESIG, and E of the opposite phasedeveloped across secondary 81. These A.-C. comparison voltages arerectified and filtered by the combination of rectifiers 82 and 83, inseries with the two secondaries 81 and 81, respectively, and bycapacitors 84 and 85, connected across the series combination of thesecondary coils and the rectifiers, The D.-C. voltages of these circuitsare developed across a pair of resistors 86 and 87, thus furnishing apair of voltages labelled E The positive end of resistor 86 is connectedto filament or cathode 88 of thyratron 35, while the positive end ofresistor 87 is connected to filament or cathode 89 of thyratron 36.

In order to provide for appropriate control of the thyratron grid tocathode circuits, it remains to provide an A.-C. bias voltage, and aD.-C. bias voltage, for each circuit. To provide these voltages, a pairof transformers 90 and 91, having primaries 92 and 93, respectively, areconnected to the usual power mains, which may conveniently supply 115volts, 6O cycle power. The secondaries 94 and 95 of transformers 90 and91 are center tapped at 96 and 97, respectively. A potentiometer 98 isconnected directly across the lower half of secondary 94 of transformer90, while a potentiometer 99 is connected directly across the upper halfof secondary 95 of transformer 91. Consequently, A.-C. bias voltages aredeveloped across these potentiometers, and the levels of bias voltage tobe used in the grid-cathode circuits of the triodes are selected bysliders 109 and 181 of potentiometers 98 and 99, respectively. It willbe noted that opposite halves of secondaries 94 and 95 are used todevelop the A.-C. :bias voltages, so that the two A.-C.'voltagessupplied to thyratrons 35 and 36 will be of opposite phase.

To provide the D.-C. bias voltage for each of the thyratrons, the otherhalves of secondaries 94 and 95 are provided with rectifying andfiltering elements including rectifiers 110 and 111, respectively, andcapacitors 112 and 113. The shunt-connected filter capacitors 112 and113 are connected in parallel with potentiometers 114 and 115,respectively, thus developing a D.-C. voltage thereacross. Theconnections of rectifiers 110 and 111 are such that the sliders 116 and117 of potentiometers 114 and 115, respectively, are negative withrespect to the cathodes of the corresponding thyratrons. Potentiometerslider 116 is connected through an appropriate voltage droppingresistance 118 to control grid 37 of thyratron 35, and potentiometerslider 117 is connected through a similar resistor 120 to control grid38 of thyratron 36.

As a result of. these connections, the grids of thyratrons 35 and 36 areboth driven in the negative direction by a D.-C. signal voltage, Eobtained by comparison of the A.,-C. signal voltage and the A.-C.reference voltage and rectification of the difference voltage, and thegrids are driven in the positive direction by a D.-C. bias voltage, E toadjust the bias of the thyratrons to appropriate levels with respect totheir critical voltages. The grids are further driven by A.-C. biasvoltages, e,, of opposite phase.

In order to provide plate voltage for thyratrons 35 and 36, a pair oftransformers 120 and 121, having their primaries connected to the powermains source of 60 cycle voltage, are provided. The secondaries oftransformers 120 and 121, 122 and 123, respectively, each have 9. Of itsends connected. to the corresponding plate of thethyratrons and itsopposite end connected to the cathode or filament thereof. Theconnections and windings are so arranged that plate 125 of thyratron 35is positive with respect to its cathode when plate 126 of thyratron 36is negative with respect to its cathode. The end of secondary 122opposite the plate-connected end is connected to one brush 130 of thedome drive motor 21, while the other brush 131 of the motor is connectedto the cathode of thyratron 35. Likewise, the end of secondary 123opposite the plate-connected end is connected to brush 131, while thecathode of thyratron 36 is connected to brush 130.

The field-winding 135 of drive motor 21 may be supplied with a DC.voltage from a full-wave rectifier 136 connected to the power mains.

The operation of the method and apparatus of the above-describedembodiment of the invention will now be discussed in conjunction withFig. 3. In that figure, the plate voltage of thyratron 36 is representedby curve e while the critical voltage curve of the thyratron, a curvedetermined by the design of the thyratron employed, is represented bythe dashed-line labelled critical voltage. As is well known, wheneverthe total voltage between grid and cathode crosses the critical voltageline in the negative-to-positive direction, the thyratron fires. Forthyratron 36, the voltage between potentiometer slider 117 and centertap 97 of secondary 95 is the D.-C. bias voltage, labelled B in Figs. 2and 3. This voltage tends to drive the grid of thyratron 36 in apositive direction with respect to the cathode. However, the voltageacross resistor 87, labelled E in Figs. 2 and 3, tends to drive the gridnegative with respect to the cathode and is substantially larger thanthe D.-C. bias voltage, so that the static level about which the gridvoltage varies, labelled total d-c bias in Fig. 3, is substantiallybelow the zero level and is also below the critical voltage curve. TheA.-C. bias voltage, derived between potentiometer slider 101 and centertap 97 of secondary 95, is shown as e,, in Figs. 2 and 3, and iscombined with the total D.-C. bias, so that e in Fig. 3 represents thetotal gridcathode voltage. This voltage carries the voltage of the gridwith respect to the cathode about the level determined by the algebraicsum or" E and E As is evident from Fig. 3, and in accordance withconventional practice, the A.-C. bias voltage is delayed with respect tothe plate voltage e by between 135 and 140. The magnitude of the D.-C.bias, the DC. signal, and the A.-C. bias voltages are such that, whenmotor 21. is in a position corresponding to that of theodolite drivemotor 1, the total grid voltage curve cuts the critical voltage curvefor thyratron 36 very near the end of the half cycle of the positiveplate voltage, so that tube 36 conducts current during only a smallportion of the cycle, as indicated by cross-hatching in Fig. 3. The sameis true for thyratron 35, so that the net current flowing through motor21 is zero, and no movement of the motor occurs. However, when the twodrive motors are not in synchronism, the D.-C. signal voltage changes,as a result of change in the A.-C. signal voltage. it the A.-C. signalvoltage increases in phase with the reference voltage, the D C. signalvoltage is increased, while, if the A.-C. signal voltage increases outof phase with the reference voltage, the DC. signal voltage isdecreased. The latter condition is shown in Fig. 3, where E' is shown indashed line as as being of lesser value than the steady-state or nullcondition of the D.- C. signal voltage. In such case, the total gridvoltage increased, so that the voltage level e shown in Fig. 3 as adashed curve, cuts through the critical voltage curve at an advancedpoint in the cycle, labelled the firing point out of null. Thyratron 36then carries a large amount of current, as indicated by the shadedportion of Fig. 3; In contrast, at the same time, thyratron 35 would bedriven more negative, so that it would not fire. The conduction ofthyratron 36 would cause current to pass 6 through the armature of thedome drive motor 21 in such direction as to rotate the motor tocorrespondence with the theodolite drive motor 1.

It will be evident that I have described a method and apparatus whichwill maintain the dome drive motor 21 in synchronism with the theodolitedrive motor 1, through use of a thyratron Control circuit which ispositively controlled by a signal voltage independent in phase withrespect to the thyratron plate voltage, yet without any extensiveadditions to convert the A.-C. signal voltage to be of constant phasewith respect to the thyraton plate voltage. t will further be obviousthat many minor changes could be made in the method and apparatusspecifically described herein without departure from the scope of thepresent invention. Accordingly, the invention is not to be consideredlimited to the embodiment specifically disclosed, but rather only by theappended claims.

I claim:

1. A method of controlling firing of a thyratron which comprisessupplying an A.-C.'voltage between the plate and cathode of thethyratron, comparing an AC. signal voltage with an A.-C. referencevoltage of identical frequency and of substantially constant phase andamplitude to obtain a comparison voltage, rectifying and filtering saidcomparison voltage to obtain a D.-C. signal voltage having an amplitudeand polarity determined by the amplitude and phase of the A.-C. signalvoltage as compared with the reference voltage, deriving from the sourceof said A.-C. plate voltage an A.-C. bias voltage having a constantphase delay of about l40 with respect to the A.-C. plate voltage,deriving a D.-C. bias voltage, combining algebraically said D.-C. signalvoltage, said A.-C. bias voltage, and said D.- C. bias voltage to obtaina control voltage, and supplying said control voltage between grid andcathode of said thyratron.

2. A method of synchronizing movement of one motordriven device withanother motor-driven device which comprises supplying an A.-C. signalvoltage of amplitude and phase dependent upon the relative position ofmotor of said other motor-driven device with respect to the position ofthe motor of said one motor driven device, comparing said A.-C. signalvoltage with an AC. reference voltage of frequency identical with saidsignal voltage and of substantially constant amplitude and phase, toobtain a comparison voltage, rectifying said comparison voltage toobtain a DC. signal voltage of amplitude dependent upon the relativeamplitudes and phases of said A.-C. signal and said A.-C. referencevoltages, supplying the plates of a pair of thyratrons with an A.-C.plate voltage in such fashion that the plate voltages are substantially180 out of phase with each other, deriving from the source of said platevoltage an A.-C. bias voltage having a phase delayed with respect to theplate voltage, combining algebraically said D.-C. signal voltage andsaid A.-C. bias voltage in one sense to obtain one composite voltage andin the opposite sense to obtain another composite voltage, applying saidone composite voltage between grid and cathode of one thyratron andapplying said other composite voltage between grid and cathode of theother thyratron, and supplying the plate currents of said thyratrons inopposite direction to said motor of said one motor-driven device.

3. The method of claim 2 including the steps of adding to each of saidcomposite voltages a D.-C. bias voltage in such sense and of suchamplitude to maintain the grid of each of the thyratrons negative withrespect to the cathode, but within approximately the peak amplitude ofthe A.-C. bias voltage from the thyratron critical voltages.

4. A thyratron control system comprising means for supplying an A.-C.signal voltage, means for supplying an A.-C. reference voltage of thesame frequency as the signal voltage and of substantially constantphase, means for adding said A.-C. signal voltage and said A.-C.reference voltage to obtaina composite voltage, means for rectifyingsaid composite voltage to obtain a -D.-C. signal for supply between gridand cathode of said thyratron.

5. A thyratron control system as defined in claim 4 including means forsupplying a D.-C. bias voltage and for adding said D.-C. bias voltagealgebraically with said pulsating control voltage and said D.-C. signalvoltage between grid and cathode of said thyratron.

6. A thyratron control system comprising means for supplying an A.-C.signal voltage, means for supplying an A.-C. reference voltage havingthe same frequency as the A.-C. signal voltage, means for adding theA.-C. signal voltage and the A.-C. reference voltage of one phase toobtain one composite A.-C. voltage, means for adding the A.-C. signalvoltage and the A.-C. reference voltage of the opposite phase to obtainanother composite A.-C. voltage, means for rectifying both compositevoltages to obtain one D.-C. signal voltage and another D.-C. signalvoltage, a'pair of thyratrons, means for supplying an A.-C. voltage ofone phase between plate and cathode of one of said pair of thyratrons,means for supplying an A.-C. voltage of the same frequency and oppositephase between plate and cathode of the other of said pair of thyratrons,means for supplying an A.-C.

ias voltage of constant frequency and phase relation to said A.-C. platevoltages, means for combining said one D.-C. signal voltage and saidA.-C. bias voltage algebraically to obtain one control voltage and forsupplying said one control voltage between grid and cathode of onethyratron, and means for combining said other D.-C. signal voltage andsaid A.-C. bias voltage to obtain another control voltage and forsupplying said other control voltage between grid and cathode of saidother thyratron.

7. A thyratron control system as defined in claim 6 including means foradding to each of said control voltages a D.-C. bias voltage.

8. An apparatus for causing the motor of one motordriven device to trackwith the motor of another motordriven device which comprises means forderiving an AEC. signal voltage of phase and amplitude indicative of thediiference in relative positions of said two devices, means forsupplying an A.-C. reference voltage of the same frequency as the A.-C.signal voltage and of substantially constant amplitude and phase, meansfor comparing said A.-C. signal voltage with said A.-C. referencevoltage of one phase to obtain one comparison voltage, means forcomparing said A.-C. signal voltage with said A.C. reference voltage ofthe opposite phase to obtain another comparison voltage, means forrectifying and filtering said one comparison voltage to obtain one D.-C.signal voltage, means for rectifying said other comparison voltage toobtain another D.-C. signal voltage, a source of A.-C. voltage ofsubstantially constant phase and frequency, means for deriving from saidsource an A.-C. bias voltage, means for combining algebraically saidA.-C. bias voltage of one phase and said one D.-C. signal voltage toobtain one control voltage, means for combining algebraically said A.C.bias voltage of the opposite phase and said other D.-C. signal voltageto obtain another control voltage, a pair of thyratrons, means forsupplying said one control voltage between grid and cathode of one ofsaid thyratrons, means for supplying said other control voltage betweengrid and cathode of the other of said thyratrons, the grid-cathodecircuits of the voltage a second A.-C. plate voltage of opposite phaseto said first A.-C. plate voltage and for supplying said second platevoltage between plate and cathode of said other thyratron, and-means forsupplying the difference of the plate currents of said pair ofthyratrons to the motor of said one motor-driven device.

9. The apparatus of claim 8 including means forsup plying a DJ. biasvoltage between grid and cathode of each of said thyratrons of suchpolarity and amplitude as, when added into said control voltages, tomaintain the grid of each thyratron negative with respect to thecathode.

10. T e apparatus of claim 9 in which both said cornparing meanscomprise a first pair of grid-containing vacuum tubes, 21 second pair ofgrid-containingvacuum tubes, and a pair of transformers havingcenter-tapped primary coils, said A.-'C. signal voltage being appliedbetween control grid and cathode of each of said first pair of vacuumtubes, said A.-C. reference voltage being applied in opposite phasebetween grid and cathode of the two of said second pair of vacuum tubes,the plate of one of said first pair and the plate of one of said secondpair of vacuum tubes being connected to opposite ends of one primary ofsaid pair of transformers, the plate of the other of said first pair andthe plate of the other of said second pair of vacuum tubes beingconnected to the opposite ends of the other primary of said pair oftransformers, and the center taps of said primaries being connected tothe cathodes of all of said tubes through a source of plate voltage.

11. The apparatus of claim 8 which said means for deriving an A.-C.signal volt-age comprises a control transmitter-type synchro having itsrotor driven with the motor of said other motor driven device, saidsynchro having its rotor coil supplied with voltage from said means forsupplying an reference voltage, and a control transformer-type synchrohaving its rotor driven with the motor of said one motor-driven device,the respective stator coils of said two synchros being connected to eachother and the rotor coil of said control transformer-type synchrothereby developing said A.- C. signal voltage across it.

12. A method of controlling firing of a gaseous discharge tube for whichthe signal voltage need not be of the same frequency and phase as thetube plate voltage which comprises comparing the signal voltage with anA.C. reference voltage of identical frequency and of substantiallyconstant phase and amplitude to obtain .a comparison voltage, rectifyingand filtering said comparison voltage to obtain a D.-C. signal voltagehaving an amplitude and polarity determined by the'amplitude and phaseof the A.-C. signal voltage as compared with the reference voltage,deriving from the source of tube plate voltage an A.-C. bias voltage ofthe same frequency as the plate voltage and having a controllable phaserelation therewith, and applying the algebraic sum of the D.-C. signalvoltage and the AC. bias voltage between the control electrode andcathode of the discharge tube.

References Cited in the file of this patent UNITED STATES PATENTS1,944,756 Quarles Jan. 23, 1934 2,115,686 Riggs Apr. 26, 1938 2,175,009Anderson Oct. 3, 1939 2,254,899 Laubenheimer Sept. 2, 1941 2,431,501Phillips Nov. 25, 1947 2,516,144 Pawley July 25, 1950 2,632,872 WarsherMar. 24, 1953 2,774,928 Johnson et al. Dec. 18, 1956 OTHER REFERENCESElectronics in Industry, Cl'1ute,,G. M.,'vol. 1, pp, 193, McGraw-Hill,1946. v g

